The present invention generally relates to microwave discriminator circuits and, more particularly, to an unequal stub length diplexing microwave frequency discriminator circuit.
Discriminator circuits have various usages, the most common of which is for demodulation of frequency-modulated signals. A variety of discriminator circuit types have been disclosed over the years.
One discriminator circuit type which uses two different frequency-sensitive portions tuned to different center frequencies, with a detector diode attached to each portion, is similar to a conventional Round-Travis discriminator circuit. Another discriminator circuit type is similar to a conventional Foster-Seeley discriminator circuit. The Foster-Seeley discriminator circuit type splits power and uses differential line lengths and power recombination to produce two different frequency selectivities by phasing. Low frequency versions of these two named circuit types have been used in FM radio receivers for years. See for example, the "Radiotron Designer's Handbook", 4th Edition, edited by F. Langford-Smith, 1953, reproduced and distributed by RCA, Harrison, NJ.
A review and analysis of the microwave version of the Foster-Seeley discriminator circuit type, based on broadband 90.degree., -3 dB hybrids and differential line lengths, may be found in "MIC 7-to-11 GHz Frequency Discriminator", by U.H. Gysel and J.P. Watjen, Stanford Research Institute, Final Report, AD/A034977, September 1976. In contrast to this broadband design, current interest centers on microwave discriminator circuits which operate in this same general area, but have superior linearity over a much more restricted bandwidth on the order of five percent maximum.